Ferroelectric liquid crystal materials are of important application to flat panel liquid crystal array devices because of their high switching speed and bistability. Unlike supertwist nematic liquid crystal devices, for example, the pixels of such a device will remain in a particular state without continued application of a particular drive voltage. In a large area panel display device which has to be addressed by multiplexing this is a significant advantage. Ferroelectric liquid crystal arrays and a driving scheme therefor are described in `The JOERS/Alvey Ferroelectric Multiplexing Scheme` published in Ferroelectrics, 1991, Vol.122 pages 63 to 79. In such driving schemes a liquid crystal array has a first and second set of driving electrodes arranged at right angles to each other defining a matrix. A plurality of pixels are defined at the intersection of an electrode from the first plurality and an electrode from the second plurality. However, by the very nature of this layout, it is not possible to address each pixel individually. The type of addressing scheme used most commonly applies a strobe signal in sequence to one of the sets of electrodes (referred to hereafter as the row electrodes) while applying the relevant data signals for the currently-strobed row to the second set of electrodes (hereafter referred to as the column electrodes).
One consequence of such a scheme is that the data signals applied to the column electrodes are applied to every pixel in the respective column, even though only one pixel in the column is actually being addressed at any one time. In a ferroelectric display it is not feasible to remove such signals (for example by open-circuiting the non-strobed row column electrodes) because they are required to apply an AC stabilisation signal to the pixels of the array. Such a signal prevents the liquid crystal molecules in the array relaxing to a position which has an unfavourable optical performance. These signals, however, are continually applied at a high frequency to every column electrode to drive a capacitive load including the pixels of the device. The column electrodes generally include transparent indium tin oxide (ITO) tracks which have a certain resistance so the charging and discharging of the pixels dissipates power in these tracks which heats the device.
The temperature of the device is particularly critical in a ferroelectric liquid crystal array device because of the large temperature sensitivity of ferroelectric materials themselves. To some extent effects of global temperature changes to the device can be compensated for in the addressing waveforms. For example changes in the switching speed (operating region) can be compensated for by changing the shape or amplitude of the strobe voltage, whilst changes in the angle of the director in an AC stabilised position can be compensated for by changing the amplitude of the column (data) waveforms. However, the prior art drive schemes such as the one described in the reference above, apply rectangular waves to the column electrodes to drive the device and these waveforms have a rich harmonic content including substantial frequency components at high multiples of the fundamental frequency. Since each column of the array appears as a distributed RC ladder to the driving circuitry, these higher harmonics of the driving waveform are attenuated heavily by the device and the highest attenuation occurs at the driven end of the column electrodes, in other words at the edge of the device. This causes non-uniform heating of the device that cannot be compensated by adjusting the row or column signals (since they clearly apply to all of the pixels in a column). The consequence of this is variations in contrast or colour over the array display device (or, in extreme cases failure to switch when addressed) which is unacceptable. Liquid crystal devices based on nematic liquid crystal phases do not suffer from these problems because of their higher tolerance of temperature variations.